1. Field of the Invention
This invention relates to information recording and more particularly, to high density information records of a variable electrostatic capacitance type, such as video or digital audio discs, on which an information signal is recorded as geometric variations.
2. Description of the Prior Art
In certain information playback systems of the electrostatic capacitance type, an information signal is recorded as geometric variations by forming pits in a spiral plane or groove or in concentric planes or grooves, thereby obtaining high density information records such as video or digital audio discs. The individual pits of the record are very small and are arranged in a very high density. When the geometric variations are tracked with a pickup stylus such as a diamond having an electrode, capacitive variations are established between the stylus and the record according to the geometric variations, so that the recorded information signal can be played back.
Several kinds of information signal records of the electrostatic capacitance type have been proposed for use in the above type of electrostatic capacitance playback system. In one such record medium, an electrode is provided on one or both surfaces of a record substrate on which geometric variations are press molded according to an information signal. This permits formation of an electrostatic capacitance to be established between the record electrode and an electrode of a pickup stylus. More particularly, the record substrate is covered with a thin metallic film having a thickness of several hundred angstroms on which pits are formed as desired, and also with a several hundred angstrom thick dielectric layer overlying the metallic film. The dielectric layer serves to prevent short circuiting of the electrodes and increase the dielectric constant between the electrodes. However, the record of this type requires a number of fabricating steps including press molding the record substrate, depositing the thin metallic film and the dielectric layer, and the like. Thus, the fabrication is complicated and troublesome with the need of a relatively large-scale manufacturing apparatus. Thus, production cost become very high.
Another type of known information record of the electrostatic capacitance type is one which is obtained by press molding conductive plastic compositions comprising polyvinyl chloride resins, lubricants and several tens percent of carbon black on which the information signal is recorded as geometric variations. In this type of record, an electrostatic capacitance is established between the electrode of a pickup stylus and the record itself. Thus, no steps of depositing a thin metallic film on the substrate are needed. In addition, because fine particles of carbon black are individually covered with the resin, the dielectric film is not necessary. Thus, this type of record can be simply manufactured at relatively lost costs.
However, when the record made of the known conductive resin composition comprising vinyl chloride resins and carbon black is set in a playback system and played back, dropout defects may be often experienced.
We made studies in order to clear up the causes of the dropout defects and found that the records of the electrostatic capacitance type obtained from conductive resin compositions comprising vinyl chloride resins and carbon black involved blisters by which the dropout defects were produced.